This disclosure pertains to quantum dots, and particularly to a quantum dot based radiation source, a radiometric calibrator using the quantum dot radiation source, and a method of calibrating a detector using the quantum dot based radiation source.
Radiometric calibration is utilized to associate or link intensities measured by a sensor or detector (such as intensities measured by each pixel of a charge coupled device (CCD)) to a physical parameter. For example, radiometric calibration allows comparison of wavelength spectral intensities from different radiation sources. Radiometric calibration may be needed as the photon flux distribution in the unprocessed detected wavelength spectrum can be different from the photons flux distribution in the wavelength spectrum that is emitted by the observed object or source of radiation. This can be due to the fact that the photons detected by the sensor or detector (e.g., CCD) traversed several filters, such as the atmosphere, which can act as filter in certain regions of the wavelength spectrum, as well as filtering devices in the detector. Furthermore, radiometric calibration may be needed due to the fact that the detector or sensor quantum efficiency is not flat across the detected wavelength spectrum.
As a result, some degree of radiometric calibration may be needed by electro-optical instruments, such as electro-optical instruments used for remote sensing (e.g., space-based remote sensing instruments) to establish a relationship or link between the signal output by the instrument and the photon flux emitted by the object under study. For example, radiometric calibration may be used to know how a pixel's “dark number” correlates to a fixed unit of illumination (W/cm2 or photons/cm2-s). Furthermore, radiometric calibration can also provide meaningful comparison of different observed phenomena.
Current radiometric calibration is performed primarily by using a bright filament-based lamp as a radiation source. For example, in order to simulate the blackbody output of the sun, the filament or filaments in the lamp are operated at relatively high temperatures (e.g., approximately 3500 K or more). This creates a number of problems in terms of heat dissipation, power consumption, volume and reliability. Furthermore, the use of a lamp wherein the filament is operated at a relatively high temperature can pose additional challenges. Indeed, the lamp must be mechanically manipulated with an aperture wheel to achieve a desired dynamic range. In addition, the radiation output of the lamp can vary (e.g., degrade) over time due to changing electrical loads or due to changes in filament characteristics. Also, this type of radiation source is generally not stable over its useful life span.
Instead of filament-based lamps, light emitting diodes (LEDs) have also been used for radiometric calibrations. LEDs are generally more stable than filament based lamps. However, LEDs emit in narrow wavelength ranges and, as a result, can only cover a limited portion of the radiation spectrum.
Other light sources used for radiometric calibration include the use of solar diffusers which usually require checks by other sources (ground images). However, the solar diffusers degrade over life due to material degradation, contamination or mechanical problems. The use of solar diffusers for calibration include calibrating ex-situ by looking at features on the ground that have a known reflectivity. For example, the instrument stares at a patch of snow on the earth. However, these methods have limitations, since the reflectivity of the material (e.g., snow on the ground surface) can change over time and thus affect the reflected spectrum.
This disclosure addresses various issues relating to the above and other issues with conventional approaches.